1. Field of the Invention
The present invention relates to mobile device tracking and more particularly to determining the location of a mobile device using targets which encode information.
2. Background to the Invention
A common scenario in which location of a mobile device is obtained for the purposes of tracking is in a modem store where the mobile device which is tracked is a shopping cart. A cart may be tracked for a variety of reasons, for example, to obtain knowledge of the flow of customers through a store to use when arranging products in the store to avoid bottlenecks and ensure maximum exposure of specific products selected for promotion. For example, knowledge of the current location of a cart is used to provide information to the customer using the cart, which is pertinent to that location in the store, such information being provided to a display device associated with a shopping cart.
One known method of tracking the location of a shopping cart in a store uses powered infra-red beacons which are attached at regular intervals to the ceiling of a store. Each beacon transmits a burst of infra-red which encodes a unique zone identification. The infra red signal is then received by an infra-red receiver mounted on a shopping cart thus enabling zone in which the shopping cart is located to be determined. However, there are several problems with this method. For example, the beacons require power and as a result it is necessary to either replace batteries in each beacon at regular time intervals or install an expensive wiring harness connected to an electricity supply. Further store lighting and/or lighting ballasts can cause interference with the infra-red receiver on the cart, and also the granularity of the location depends on the distance between the beacons.
An alternative known method of tracking the location of a shopping cart in a store uses one or more wireless LAN (WLAN) access points and the location of a cart is calculated using the signal strength for the various access points measured by a receiver located on the shopping cart. However this method also has known problems which include metal in the store environment causing reflections and multi-path, reducing accuracy of position measurement such that additional access points and directional antenna may have to be added to the store to provide sufficient spatial resolution and robustness. Further the received signal, and calculated cart position, may be affected by antenna orientation (rotation) and also the access points require a power connection.
Position related activities and tracking in respect of mobile devices is also preformed in other fields in which it is known to use, for example, electronic tags, RFID tags, or barcodes for this purpose.
For example, U.S. Pat. No. 5,076,690 to deVos, et al., discloses position sensing based on the triangulation calculation using at least three retro-reflective elements spaced apart and positioned at known locations. One of the retro-reflective elements may be a bar code so that it is identifiable. A rotating member with a light transmitting and detecting device is used to locate and illuminate the retro-reflective elements. However this solution is not suitable, for example, for determining cart location in a store because it requires a line of sight to at least 3 wall mounted reflectors which would be difficult to achieve for all possible cart locations in a large store.
For example U.S. Pat. No. 5,812,267 to Everett, et al., discloses determining a distance from multiple reflectors using two sensors each of which each generate light and detect a reflected light signal from a reflector. Triangulation is then used to determine a distance from the reflectors based on the detected signals from each sensor. However, this solution is not suitable, for example, for determining cart location in a store because it calculates only a distance from the reflectors and cannot distinguish one reflector from another. As a result it is only possible to calculate location based on a known previous location which is not appropriate for a shopping cart.
For example U.S. Pat. No. 4,918,607 to Wible, et al., discloses position sensing based on the triangulation calculation using reflection from two retro-reflective elements each of which encode their location using a bar code. A scanning process is used to locate the retro-reflective elements which are positioned in pre-selected spaced apart co-linear locations. However, this solution is also not suitable, for example, for determining cart location in a store because the scanning mechanism scans only selected areas and restricts the locations into which the retro-reflective elements must be placed.
Further in the art it is known to use retro-reflective barcodes in guidance systems. For example U.S. Pat. No. 5,202,742 to Frank, et al., discloses a laser radar apparatus for vehicle guidance along a road. Light propagation delay from a reflective target is measured to determine target distance and a scanning mirror used to measure the angle between the laser apparatus and the reflective target. The trigonometric relationship of the target distance and angle are then used to calculate distance from the target. The reflective targets may incorporate a bar code which imparts information such as the side of the road, an approaching exit, speed limit changes etc. Further, for example U.S. Pat. No. 4,790,402 to Field, et al., discloses a guidance system in which a lateral scanning laser transmitter-receiver detects retro-reflective targets along a path and these are used to keep the vehicle on a prescribed path. The retro-reflective targets may include perpendicular bar-codes to make them easier to identify and read by the scanner.
However, guidance systems do not provide a solution suitable, for example, for determining cart location in a store in which a shopping car can be moved unpredictably around a store. This is because, for example, the guidance systems calculate a position of a vehicle relative to a target, and this information is then used to locate the next target and maintain the vehicle on a prescribed path. Accordingly if the vehicle loses sight of a next target it becomes lost and cannot re-determine its position based on a subsequent detection of another target. However because a customer with a shopping cart does not generally follow a prescribed path it is necessary to determine a location based on detection of a single target.
Accordingly none of the known art in related fields provides a solution to the problems associated with the current solutions to determining position of shopping cart.